In an optical communication system, an optical receiver module converts optical signals into electronic signals, and its performance directly affects performance and stability of an optical communication system.
FIG. 1 shows a conventional analog optical receiver module with analog adjustment, which mainly includes three parts.
(1) An optical-electronic conversion circuit 11, which has an optical signal input and a weak electronic signal output, adopts a PIN-Photodiode detector or an Avalanche Photodiode detector (APD) or an optical detector; and a limiter amplifier (LIA) 13 amplifies the weak electronic signal limitedly to extract data and clock signal.
(2) A bias voltage adjusting circuit provides a bias voltage to the optical detector. Different APD need different bias voltage, and temperature compensation is also needed when an APD is working at different temperature, so the bias voltage should be adjustable. There are also a DC/DC voltage boost circuit 12, a sampling resistance RS, a regulation resistance RP2 of the DC/DC voltage circuit, a thermistor RT. In this diagram, the temperature compensation mode is analog.
FIG. 2 shows a typical and conventional bias voltage adjusting circuit diagram. With an integrated circuit (IC), the DC/DC voltage boost circuit is connected to a regulation resistance RP2 to regulate the voltage at point FB and finally to change the higher voltage V_APD (35˜−75 V) through a feedback voltage Vfb in the IC.
FIG. 2 shows that V_APD=Vfb×R51÷RP2.
It is necessary to provide different temperature compensation i.e. different bias voltages, for different optical detectors working at different temperatures in order to obtain a higher receiving sensitivity; the compensation coefficient is about +0.08V/ ° C.˜+0.15V/° C. In convention, a temperature transducer connected externally is used, and its output voltage regulates the feedback voltage Vfb to obtain different V_APD bias voltages; but during in use, it is impossible to track the bias voltage V_APD, namely it is unable to compensate fully different APD temperature characteristics. In order to have a better temperature compensation for each APD, what has been done presently is to adjust the potentiometer RP2 according to parameters of each APD and then to measure bias voltage V_APD with tools, such as a multimeter.
(3) Voltage output circuit of optical power detection 14 measures a bias current passing through the optical detector to detect received optical power, and the bias current is measured by the voltage in the sampling resistance RS.
FIG. 3 shows a conventional zero-adjustment circuit that provides a dark current compensation for a detected optical power. In this circuit, RS is a sampling resistance of the bias current; 31 is a PIN photodiode or APD; RP3 is a zero-adjustment resistance of the dark current; and the operational amplifier OP2 outputs an Optical Power Measurement voltage (OPM). Through the zero-adjustment resistance RP3, the output voltage of circuit 14 is adjusted to implement a zero-adjustment when there is no optical signal inputted, i.e. dark current zero-adjustment compensation.
At present, many optical receiver modules have a Loss Of Signal (LOS) upward reporting function, which is implemented by a hysteresis comparison circuit. Usually, comparing a peak-to-peak value of a sampling signal in the limiter amplifier chip or the measured output voltage of the optical power detecting circuit with a preset voltage, and when the receiving optical power sensitivity is lower than 3 dB, it is reported; the preset voltage might be set by a potentiometer, and it is fixed after setting.
As the optical receiver module mentioned above is mature technically, it is popularly used on the optical communication network. Nevertheless, adjustment of each parameters in this module is all implemented by potentiometers, and a potentiometer itself has disadvantages, such as low reliability, parameter drift caused by age etc.; besides, the parameters cannot be monitored and adjusted in real time and adjustment of parameters can only be done manually, which depends on skills of workers. In this way, production efficiency is low and non-linear compensation characteristic is bad.
In summary, the optical receiver module mentioned above has disadvantages as followings.
(1) Adjustment procedure of parameters is complicated and expensive, and maintainability of parameters online is bad.
Accuracy and stability of an optical receiver module with analog adjustment mainly depend on the accuracy and stability of a potentiometer, and a potentiometer changes its resistance value with mechanical contacts, so an optical detector working point is changed by the mechanical contacts of a potentiometer. Since a mechanical contact has problems of contact fault and bad temperature characteristic, also during transportation and vibration a mechanical contact will drift, so the parameters of an optical receiver module are drifted too.
Adjusting and testing a potentiometer is a complicated procedure, and this makes it difficult to put the optical receiver module production into automation, so it is difficult to reduce the production cost.
Adjusting and testing an optical receiver module highly depend on skill of a worker, so it is difficult to keep consistence of products.
Parameters of an optical receiver module with analog adjustment are difficult to reserve in a database which is set up with an IT platform, but this database is important for product maintenance and improvement.
Aging will cause drift of parameters, and in an optical receiver module with analog adjustment, it is impossible to make an online adjustment of the parameters without interrupting services, so the reliability and stability cannot well satisfy telecommunication equipment requirement.
(2) Parameters cannot be stored and sampled expediently, and the received optical power cannot be read out directly.
There is no any memory to store parameters, such as the initial bias voltage at adjustment, receiving sensitivity and the date of adjustment etc., so it is impossible to obtain those parameters online.
As to optical power detection, output voltage can be measured, but the received optical power cannot be read out directly.
(3) Nonlinear compensation is bad.
The temperature compensation mainly depends on a thermistor, and actual compensation is inconsistent with that of a ADP because of the thermistor temperature characteristic, so that key performances of the module such as receiving sensitivity get worse.
Optical power is measured through detecting response current of ADP and dark current alters with environment temperature so that precision of detection descends.
Along with development of digital technology, the analog optical receiver configuration should be replaced by a digital technique.